Wireless communication method and apparatus for optimizing access point channel selection

ABSTRACT

A wireless communication method and system for optimizing channel selection for an AP. The channel selection optimization process includes four sub-processes: 1) a measurement process; 2) a candidate channel determining process; 3) a channel selection process; and 4) a channel update process. Candidate channels used for supporting communication performed by the AP are determined. The candidate channels are chosen from an allowable channel set (ACS) if a detected interference of each candidate channel is less than an established maximum allowed interference.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims priority from U.S. Provisional Application Nos.60/526,134 filed on Dec. 1, 2003 and 60/535,447 filed on Jan. 8, 2004,which are incorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to a wireless communication system. Moreparticularly, the present invention relates to selecting the mostappropriate operating channel for an access point (AP).

BACKGROUND

The conditions of a radio link under which a wireless communicationsystem operates may change at any time. Since a wirelesstransmit/receive unit (WTRU) is mobile, the WTRU may be out-of-range, orwithin range of one or more APs depending upon the position of the WTRU.

The capacity of a communication system is sometimes limited due tobandwidth considerations. The bandwidth capacity of the communicationchannel, or channels, available to the communication system tocommunicate data is finite, and must be shared among a plurality of APsand portable WTRUs.

There are several current schemes that are employed in order to increasethe capacity of a wireless communication system. Channel, i.e.,frequency, selection is one of such schemes, whereby one or more APs ina network select one or more channels to communicate with theirassociated WTRUs. Coordination of AP channel selection is usuallyperformed manually. However, it is very impractical to manuallycoordinate channel selection in response to every small change in thenetwork configuration since it may cause a redesign and reconfigurationof all APs. Unlicensed spectra and external sources of interference alsoraise problems that are not adequately addressed by manual coordination.Moreover, it is difficult for manual channel selection to assignchannels such that the traffic loads of neighboring APs are shared amongthe available channels in a way that maximizes overall system capacity.

Another problem with prior art schemes is encountered when a multipleAPs attempt to power-up simultaneously. When this occurs within anetwork, all of the APs try to make a channel selection at the sametime. Thus, the channel selection by the APs would not be optimal sinceeach AP does not take into account the channel selection of neighboringAPs.

A method and apparatus which automatically optimizes channel selectionto avoid the above-mentioned problems associated with known manualchannel selection processes would be greatly beneficial.

SUMMARY

The present invention is related to a wireless communication method andapparatus for optimizing channel selection for an AP. The apparatus maybe an AP and/or an integrated circuit (IC).

The channel selection optimization process includes foursub-processes: 1) a measurement process; 2) a candidate channeldetermining process; 3) a channel selection process; and 4) a channelupdate process. Candidate channels used for supporting communicationperformed by the AP are determined. The candidate channels are chosenfrom an allowable channel set (ACS) if the detected interference of eachcandidate channel is less than an established maximum allowedinterference.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of a wireless communication system inaccordance with the present invention;

FIG. 2 is a flow diagram of a channel optimization process according toone embodiment of the present invention; and

FIGS. 3A and 3B, taken together, are a detailed flow diagram of achannel selection process in accordance with another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the terminology “WTRU” includes but is not limited to a userequipment (UE), mobile station, fixed or mobile subscriber unit, pager,or any other type of device capable of operating in a wirelessenvironment.

Hereinafter, the terminology “AP” includes but is not limited to anaccess point, a base station, Node-B, site controller, or any other typeof interfacing device in a wireless environment.

The features of the present invention may be incorporated into an IC orbe configured in a circuit comprising a multitude of interconnectingcomponents.

The present invention as described herein, is generally applicable towireless local area network (WLAN) technologies, as applied to the IEEE802.11 and ETSI HyperLAN specifications in general, but may also beapplicable to other interference-limited wireless systems such as IEEE802.15 and IEEE 802.16.

FIG. 1 is a block diagram of a wireless communication system 100according to the present invention. The wireless communication system100 comprises an AP 105 and a plurality of WTRUs 110 a-110 n. The AP 105communicates with the WTRUs 110 a-110 n over a wireless link 115 via anantenna 120. The AP 105 includes a transceiver 125, a channel selector130, a measuring unit 135, a power controller 140, a timer 145 and amemory 150. The transceiver 125 transmits signals 115 a-115 n to, andreceives signals 115 a-115 n from, the WTRUs 110 a-110 n via the antenna120.

The channel selector 130 selects a channel which is used forcommunication with each WTRU 110 a-110 n. The measuring unit 135measures operating parameters for supporting the AP 105. The measuringunit 135 is responsible for collecting, processing and storing channelmeasurements including, but not limited to: the channel utilization,(i.e., the percentage of time that the channel is busy), the level ofexternal, (non-802.11), interference, the received signal strengthmeasured on received packets, and the like. The power controller 140controls the transmission power of the AP 105. The timer 145 sets one ormore predetermined periods during which the AP 105 performs certainoperations. The memory 150 provides storage for the AP 105, includingrecording data such as results of the measurements.

FIG. 2 is a flow diagram of the channel optimization process 200according to the present invention. Channel optimization refers to theprocess of choosing the best channels, (i.e., frequencies), that aparticular AP or a network of APs uses under particular trafficconditions. Channel optimization may be implemented either manually orautomatically, and can be initiated at deployment or performeddynamically during operation. The channel optimization process 200 maybe implemented in conjunction with wireless local area network (WLAN)applications, e.g., in accordance with IEEE 802.11.

As shown in FIG. 2, the channel optimization process 200 begins at step205. The channel optimization process 200 dynamically determines theoptimal operating channel during the normal system operation, withoutexperiencing a service disruption to associated WTRUs in the BSS. Instep 210, the AP 105 periodically scans through each of a plurality ofchannels, for short periods of time, to avoid service disruption to itsassociated users, and to take measurements on these channels. If, instep 215, it is determined that the AP is operating in a period of lowload, i.e., no BSS traffic and/or no associated users, the AP 105invokes the channel optimization process to determine if a new channelis more suitable by calculating the “predicted channel load” for eachchannel based on measurements taken during a high system load (step220). In step 225, the AP 105 changes its operating channel to thechannel with the lowest predicted channel load.

In current IEEE 802.11 networks, there is no mechanism for the AP 105 tonotify associated WTRUs 110 of a change of operating channel, (at leastnot in the basic standard). If an AP 105 changes channel, each of itsassociated WTRUs 110 will eventually realize that they have lostcommunication with the AP 105, and will eventually begin a search for anew AP. They will probably reselect the same AP on its new operatingchannel. The problem, however, is that the WTRUs will perceive a serviceinterruption from the time they lose communication with the AP to thetime they re-associate with it on the new channel. In order to avoidservice interruption, channel optimization process 200 waits until thereis no traffic in the BSS (cell) to change channels. On the other hand,some versions of the IEEE 802.11 standard, (namely IEEE 802.11h andpossibly a future version of the standard), may allow for the AP tosignal to its WTRU to change channels. In this case, the channeloptimization process 200 does not have to wait until there is no BSStraffic. Thus, the channel optimization process 200 can be runperiodically and change operating channels whenever needed.

In all cases, the channel optimization process 200 scans a sequence ofchannels, (e.g., a list of channels 1-11), to detect the best channelavailable. The channels may be scanned in a predetermined order, or thechannels may be scanned randomly. It is important to note that thechannel scanning does not start when there is no BSS traffic. Thechannel scanning occurs continuously throughout the normal operation ofthe AP 105. For example, every 0.5 seconds the AP 105 may listen to adifferent channel for 5 ms. The AP may repeat this periodically, eachtime scanning a different channel. By doing so, the AP 105 steals 1%, (5ms every 500 ms), of the medium time to scan other channels, resultingin very little impact to the associated users. The channel sequence doesnot need to include all available channels. Information associated witheach AP detected on each channel is recorded. This information mayinclude, but is not limited to, the identity of other APs which areoperating on the scanned channel, an indication of whether other APs arepart of the same ESS, the signal strength of the APs, the amount oftraffic on the channel and whether there are any other sources ofinterference on the channel.

For each channel scanned, the process determines: 1) what other APs areoperating on that channel; 2) whether the APs are part of the samesystem (i.e., according to the ESS); 3) the signal strength of the APs;4) the amount of traffic on the channel; and 5) if there are any othersources of interference on the channel (e.g., non-802.11 interference).The amount of traffic on the channel is typically measured in terms ofchannel utilization, which corresponds to the percentage of time thatthe receiver is carrier locked by a WLAN signal.

The scanning is periodic and continuously occurs. Once the channeloptimization process is triggered, (i.e., when there is no BSS trafficand/or no associated users, or simply a periodic triggering mechanism,e.g., every 5 minutes), the AP 105 determines which channel provides thebest performance. This may be determined, for example, by measuringwhich channel has the least amount of interference or whether other APsare part of the same ESS. Depending on whether the other APs detectedare part of the same system, the AP can be more or less aggressive whenchoosing which channel to use.

In an alternate embodiment, coordinated channel selection may beaccomplished by: 1) having APs exchange information with each otherabout their properties (e.g. load, capabilities or position); or 2)having a centralized scheme that can obtain information from each AP,and setting the channel of all APs in the network. For the first case,the decision is still made autonomously by each AP, but the informationexchanged can allow a better decision, (e.g., it can include statisticsthat are difficult to observe externally by another AP). For the secondcase, information is gathered from the different APs and communicated toa centralized unit or device, which upon reception of the informationtakes a decision and communicates the decision back to the differentAPs.

The channel optimization process 200 is performed to choose an optimalchannel, (e.g., a less loaded channel), while a current channel is beingused. The channel optimization process 200 may be triggered by one ofseveral conditions: 1) when the last execution of the optimizationchannel selection occurred at least T_(Last) seconds ago; 2) when thereare no WTRUs currently associated with the AP; or 3) when there has beenno BSS traffic to or from the AP in the last T_(Free) seconds.Accordingly, T_(Last) is the minimum elapsed time since the lastinvocation of any of the channel selection processes for triggering thechannel optimization process 200; and T_(Free) is the minimum elapsedtime since the last BSS packet transfer to or from the AP for triggeringthe channel optimization process 200.

The channel optimization process 200 does not disrupt any ongoing datatransfers such as a voice call, web download, and/or FTP transfers, byensuring in the aforementioned triggering conditions that there is noBSS traffic for at least T_(Free) seconds prior to triggeringoptimization channel selection and that there are no WTRUs currentlyassociated with the AP 105. On the other hand, if there is a way for theAP 105 to signal a change of a channel to its associated WTRUs 110, theoptimization channel selection process 200 may run periodically withouthaving to wait for the absence of BSS traffic.

FIGS. 3A and 3B, taken together, are a detailed flow diagram of achannel optimization process 300 according to the present invention. Thechannel selection optimization 300 includes four sub-processes: 1) ameasurement process 305; 2) a candidate channel determining process 310;3) a channel selection process 355; and 4) a channel update process 380.

In the measurement process 305, the average load of each neighboringBSS, L(i), is computed. In one embodiment, the measuring unit 135periodically estimates the load of each neighboring BSS. If any loadestimate of a BSS is greater than L_(MIN), the loads of all neighboringBSSs of the estimation period are recorded in the memory 150. If allBSSs have load estimates that are less than L_(MIN), the load estimatesare ignored. Only the latest N_(load) _(—) _(est) sets of load estimatesare kept in the memory 150.

In accordance with the preferred embodiment, the AP 105 listens to oneparticular channel of an ACS at each silent measurement period (SMP).The AP 105 cycles through each channel in consecutive SMPs, and measuresan individual measurement set for each channel in the ACS. Themeasurement set contains as many SMPs as there are channels in the ACS.In a given SMP, the channel utilization (CU) of the channel is measuredby the measuring unit 135. The CU corresponds to the percentage of thetime that the transceiver 125 is carrier locked. Since CU is observedduring an SMP, all packets that cause the AP 105 to carrier lockoriginate from neighboring BSSs. The CU measurement represents theout-of-BSS channel usage. Individual CU measurements are processed inorder to obtain an average BSS load per detected BSS, B. Note that theBSS IDs of all BSSs on the channel are recorded along with each channelutilization measurement.

Only high load measurements are recorded in order to avoid unnecessarylogging. Logging corresponds to the recording or storing ofmeasurements. As described earlier, the channel optimization process 300only runs when there is no BSS traffic, i.e., the system is not loaded.In order to reduce the number of recorded measurements, the channeloptimization process 300 only stores a pre-determined number ofhigh-load measurements.

The measurements for which CU is less than C_(MIN) are eliminated inorder to ensure that channel optimization is based on measurements takenunder a significant system load. In other words, if any of the CUmeasurements performed in one measurement set is greater than C_(MIN),the entire measurement set is recorded. On the other hand, measurementsets for which all channels have a CU<C_(MIN) are ignored. Themeasurement set may be CU for each channel and the BSS IDs of all BSSson the channel.

The channel optimization process 300 determines the best channel for itsown BSS based on individual channel utilization measurements. Althoughchannel optimization should be based on measurements taken undersignificant system loads, the channel optimization process 300 can beexecuted only when the system load has lightened. In order to avoidextensive measurement logging, only the last N_(SET) of measurementwindows are kept in memory.

Referring back to FIG. 3A, in the measurement process 305, the averageload of each BSS is calculated based on the individual load of a BSS.The instantaneous load of BSS i, operating on channel k duringmeasurement set j, based on Equation (1) as follows:

$\begin{matrix}{{{L\left( {i,j} \right)} = \frac{C\left( {k,j} \right)}{N_{BSS}\left( {k,j} \right)}};} & {{Equation}\mspace{14mu}(1)}\end{matrix}$where C(k,j) represents the channel utilization and N_(BSS)(k,j)represents the number of BSSs on channel k during measurement set j. Theaverage load of BSS i is computed as the average of the instantaneousload over all recorded measurement sets, based on Equation (2) asfollows:

$\begin{matrix}{{{{\overset{\_}{L}(i)} = {\max\left( {{1\%}\;,{\frac{1}{N_{SET}}{\sum\limits_{j = 1}^{N_{SET}}\;{L\left( {i,j} \right)}}}} \right)}};}\mspace{11mu}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$where N_(SET) represents the total number of recorded measurement sets.A minimum average BSS load of 1% is imposed. The methods of computingthe average load of each BSS are not limited to the above examples.

Exemplary parameters for the alternative measurement are listed in thefollowing Table 2. As those of skill in the art would realize, otherparameters and values may be used in addition, or in place of, theseparameters and values.

TABLE 2 Default Symbol Description Type Value C(k, j) The channelutilization on channel Measurement NA k during measurement set j. Thechannel usage of a channel is defined as the percentage of time that thereceiver is “carrier locked”. This measurement is taken during a silentmeasurement period; all packets that are received at the AP originatefrom neighboring BSSs. C represents the out-of-BSS channel usage.C_(MIN) The minimum channel usage above Configuration  10% which ameasurement set is parameter recorded. N_(SET) The size of the movingwindow of Configuration 100 measurement sets that are kept in parametermemory.

In the candidate channel determination process 310, the AP 105 retrievesthe maximum allowed interference I_(MAX) (step 315), which is themaximum allowed interference on any given channel determined based onthe baseline range of an AP. Preferably, I_(MAX) for the candidatechannel determination process 310 is calculated based on Equation (3):

$\begin{matrix}\begin{matrix}{I_{MAX} = {P_{MAX} - \left( {{RNG}_{base} + {RNB}_{adn}} \right) -}} \\{{\left( {C/I} \right)_{req\_ high} - M_{I}};}\end{matrix} & {{Equation}\mspace{14mu}(3)}\end{matrix}$where (RNG_(base)+RNG_(adj)) represents the range covered by the AP and(C/I)_(req) _(high) is set to the required carrier power to interferenceratio of a high rate packet, (e.g., 11 Mbps). A margin, M₁, issubtracted to eliminate channels with interference levels too close tothe actual maximum allowed level.

A first channel is selected from the ACS (step 320). The interference Iof the channel is then measured and compared with the maximum allowedinterference I_(MAX) (step 325). If the interference I of the channel isless than the maximum allowed interference I_(MAX), the AP 105 recordsthe channel in a candidate list in memory 150 (step 330). If theinterference I of the channel is not less than the maximum allowedinterference I_(MAX), the AP 105 checks whether any more channels in theACS exist (step 335). If more channels exist, the AP 105 selects nextchannel from the ACS (step 340) and the process 300 returns to step 325.If more channels do not exist in the ACS, the AP 105 checks whether anycandidate channel is available (step 345). If, in step 345, it isdetermined that no candidate channel is available, the AP 105 increasesI_(MAX) by ΔdB (step 350), and the channel optimization process 300returns to step 320. If, in step 345, at least one candidate channel isdetermined to exist, the channel selection process 355 is performed, asshown in FIG. 3B.

The channel selection process 355 is based on the average load, L perdetected BSS and the current BSS-to-channel mapping, β(k). A predictedchannel usage, C_(PRED)(k), for all channels is computed (step 360).C_(PRED)(k) represents the predicted channel utilization on channel k,using load estimates from high load conditions. C_(PRED)(k) may besignificantly different from the most recent channel utilizationmeasurements of channel k. It is preferable to base channel selection onC_(PRED) rather than using only latest channel utilization measurements,since channel selection should be optimized for high load conditions.

For each channel, k, the average load of all detected BSSs on channel kare summed based on Equation (4):

$\begin{matrix}{{C_{PRED}(k)} = {\sum\limits_{\forall{i \in {\beta{(k)}}}}^{\;}\;{{\overset{\_}{L}(k)}.}}} & {{Equation}\mspace{20mu}(4)}\end{matrix}$

Once C_(PRED) is calculated for all candidate channels, the channel kwith the smallest predicted channel utilization is selected (step 365)based on Equation (5):K=arg _(k)min(C _(PRED)(k)).  Equation (5)

At this time, the AP 105 checks whether the selected channel k isdifferent from a current channel (step 370). If the selected channel kwith the smallest predicted channel utilization is same as the currentchannel, the channel selection process 355 ends. If the selected channelk is different from the current channel, it is determined whether thereis a significant gain in changing channels (step 375). A hysteresiscriterion H_(C) ^(Opt) ensures that there is a significant enough gainin changing channels. Specifically, the new channel is adopted if:C _(PRED)(Current _(—) channel)−C_(PRED)(K)>H _(C) ^(Opt);  Equation (5)otherwise, the optimization channel selection ends.

Exemplary parameters for the optimization channel selection are setforth in Table 3. As those of skill in the art would realize, otherparameters and values may be used, in addition, or in place of theseparameters and values.

TABLE 3 Symbol Description Type Default Value ACS Allowable channel set.Configuration {1, 6, 11} parameter T_(Last) The minimum elapsed timesince Configuration 300 seconds the last invocation of any of the FSparameter algorithms for triggering Optimization FS. T_(Free) Theminimum elapsed time since Configuration 120 seconds the last BSS packettransfer to or parameter from the AP for triggering Optimization FS.L(i) The estimated load of neighboring Internal NA BSS i. The load ofeach BSS is parameter determined every 300 seconds by the Load Balancingprocess of the Power Control algorithm. L_(MIN) The minimum load formeasurement Configuration  10% logging. If any one BSS has parameterL(i) > L_(MIN), then the load of all detected BSSs is logged. N_(load)_(—) _(est) The size of the sliding window of Configuration  2 recordedload estimates. Parameter β(k) The set of BSSs detected on channelMeasurement NA k. This is a list of the BSSs IDs that have been detectedon channel k during recent silent measurement periods. I(k) Theinterference measured on Measurement NA channel k. I is measured as theaverage received signal power in the absence of “carrier lock” by thereceiver (i.e. the receiver is not receiving any packets). RNG_(base)Baseline Range (set by the Path Internal NA Loss Discovery process)parameter RNG_(adj) Range Adjustment (set by the Load Internal NABalancing process) parameter (C/I)_(req) _(—) _(high) Minimum requiredcarrier power to Configuration  10 dB interference ratio to supportparameter maximum data rate. P_(MAX) Maximum AP transmission powerConfiguration  20 dbm parameter I_(MAX) The maximum allowed interferenceInternal NA on any given channel determined parameter based on baselinerange. M_(I) Interference margin used in the Configuration  3 dBcalculation of the maximum parameter allowable interference level,I_(MAX) Δ The amount, in dB, by which the Configuration  3 dB maximumallowed interference, Parameter I_(MAX), is increased if there are nocandidate channels for which I < I_(MAX). H_(C) ^(Opt) Hysteresiscriterion for predicted Configuration  10% channel utilization. Thedifference parameter between the predicted channel utilization of thecurrent channel and the new channel must exceed this threshold.

A simpler channel selection algorithm may be based upon only on thelogged channel utilization measurements, (i.e. by selecting the channelwith the lowest channel utilization observed during high loadconditions). However, it is likely that neighboring APs have changedoperating channels prior to invoking optimization channel selection at agiven AP. Logged CU measurements do not accurately represent channelload during the next high load period. As a result, channel selection isbased on a prediction of the channel utilization C_(PRED) which is basedon the estimated BSS load and the latest BSS-to-channel mapping.

Once the channel selection process 355 is complete, the BSS channel isupdated using a channel update process 380 if a new channel is selected.In the channel update process 380, it is determined whether any WTRUs110 are associated with the AP 105 through the current operating channel(step 385). If so, the AP 105 must first send a disassociation messageto each associated WTRU 110 (step 390). The AP 105 then changes itsoperating channel to the new channel (step 395). If there are no WTRUs110 associated with the AP 105 through the current operating channel,the AP 105 changes its operating channel to the new channel.

It is preferable that at least T_(Last) seconds elapse since the lastexecution of the channel optimization process 300. Otherwise, thetriggering criterions are ignored. Accordingly, the value of T_(Last)would be the same as the value for the channel optimization process 300.Once T_(Last) has expired since change of channel, the two triggeringconditions are evaluated periodically every T_(MEAS).

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone or in various combinations with orwithout other features and elements of the present invention.

1. A method of selecting at least an optimal channel for use by anaccess point (AP) having at least an operating channel, the methodcomprising: determining a plurality of candidate channels for use by theAP; selecting from the plurality of candidate channels a channel usedfor a new operation channel by performing the following optimizationchannel selection process: computing predicted channel utilization forall of the candidate channels; selecting from the plurality of candidatechannels a channel with the lowest channel utilization; determining ifthe selected channel with the lowest channel utilization is differentfrom a current operating channel; determining whether the channelutilization of the current operating channel is greater than the channelutilization of the selected channel with the lowest channel utilizationby at least a predefined threshold, if the selected channel with thelowest channel utilization is different from the current operatingchannel; and ending the optimization channel selection process if theselected channel with the lowest channel utilization is the same as thecurrent operating channel, or if the channel utilization of the currentoperating channel is not greater than the channel utilization of theselected channel with the lowest channel utilization by at least apredefined threshold; determining whether there are any wirelesstransmit/receive units (WTRUs) associated with the AP through thecurrent operating channel, if the channel utilization of the currentoperating channel is greater than the channel utilization of theselected channel with the lowest channel utilization by at least apredefined threshold; and disassociating all of the associated WTRUs. 2.The method of claim 1 further comprising: computing average loads ofeach of a plurality of neighboring base service sets (BSSs); andrecording information of the BSS of which average load is greater than athreshold.
 3. An access point (AP) configured to perform the method ofclaim 1.